(1) Field of the Invention
The present invention relates to a preferred semi-conducting roll such as for a charging roll, developing roll, transfer roll, etc., and also relates to a developing device in a printer, copier, facsimile machine and the like, including the rolls.
(2) Description of the Prior Art
Though not illustrated, a developing device based on electrophotography has a conductive shaft element with a supporting element layer made up of an elastic semi-conducting material layer coated on the outer peripheral side thereof and has the function of supporting a thin layer of tribo-electrified toner on this supporting element layer and developing a static latent image formed on the static latent image bearer, with this triboelectrified toner. The developing roll of this developing device employs a semi-conducting roll, which is required to have various properties such as electric conductivity, environmental resistance, low hardness, triboelectric performance, toner conveyance performance and the like.
To meet these requirements, recently studies and investigation have been made into obtaining semi-conducting rolls by using urethane rubber, NBR, silicone rubber or the like as a basic material and adding an electric conductionizer, e.g., electronic conductive material or ionic conductive material, to the basic material. In addition to meeting the requirements of the above properties and mechanical properties, this roll should meet the requirements of the initial and time-dependent printing characteristics (including print density dependent on the toner conveyance performance, fogs and reproducibility of fine lines dependent on the toner charge characteristics).
When urethane rubber is used to produce a semi-conducting roll, it is difficult to provide sufficient charge on the toner, due to its electrification characteristics. Therefore, fogs are liable to occur and the toner is easily degraded because a large amount of frictional energy acts on the toner. Further, toner conveyance is so poor that there are quite a few possibilities of causing an insufficiency in printing density. When a conventional method of producing semi-conducting rolls is used, there is the risk that the powder characteristics of the toner may be degraded and hence degrade the time-dependent printing performance.
The present invention has been devised in view of the above problems and it is therefore an object of the present invention to provide a semi-conducting roll and a developing device using the rolls, which is able to improve its resistance to abrasion, reduce density unevenness and occurrence of fogs and realize stable image forming in the long term use, without degrading the time-dependent printing performance.
In order to achieve the above object, the present invention is configured as follows:
In accordance with the first aspect of the present invention, a semi-conducting roll, at least, has one layer of an elastic semi-conducting material layer on the outer side of a conductive shaft, and is characterized in that a coating material layer containing a resin component having a SP-value of 6 to 12.5 is substantially uniformly formed on the outer peripheral surface of the semi-conducting material layer so as to control the amount of the toner adhering to the roll within the range of 0.5 to 1.5 mg/cm2.
In accordance with the second aspect of the present invention, the semi-conducting roll having the above first feature is characterized in that the coating material layer contains an electronic conductive material or ionic conductive material.
In accordance with the third aspect of the present invention, a developing device including a semi-conducting roll, at least, has one layer of an elastic semi-conducting material layer on the outer side of a conductive shaft, and is characterized in that a coating material layer containing a resin component having a SP-value of 6 to 12.5 is substantially uniformly formed on the outer peripheral surface of the semi-conducting material layer so as to control the amount of the toner adhering to the roll within the range of 0.5 to 1.5 mg/cm2.
In accordance with the fourth aspect of the present invention, the developing device having the above third feature is characterized in that the coating material layer of the semi-conducting roll contains an electronic conductive material or ionic conductive material.
According to the present invention, since the affinity between the toner and the coating material layer which supports and conveys the toner is adjusted, it is possible to control the conveyance performance of the toner and the abrasion characteristics of the roll. Specifically, the resin contained in the coating material layer is specified so as to have an SP-value of 6 to 12.5, to thereby improve the toner conveyance and durability and control the affinity between the coating material layer and the toner. Therefore, it is possible to control toner filming. It is also possible to control electrification performance by the functional groups contained in the resin. Since the toner conveyance can be controlled by the density of the surface roughness, it is possible to maintain high-quality print images free from print failures, over a long period of time.